Separation of groups of discrete particles based upon a differences in buoyancy can be accomplished by the use of a flotation tank. The tank is filled with a liquid selected so that one group of particles will sink toward the bottom, while the other rises to the surface. Suitable conveying equipment is then provided to remove the heavier particles, and those that rise to the surface can be removed by similar equipment, or by a hand skimming operation. This technique has been used very successfully in agricultural applications, particularly in the separation of a mixed mass of blueberries so that the ripe ones can be accumulated separately, and the green ones discarded. The ripe ones have a greater specific gravity, and proceed toward the bottom of the tank, where they are picked up by an inclined conveyor. Tanks for this purpose tend to be triangular in side elevation, with the conveyor supported on the inclined bottom of the tank. The mixed mass of berries is dumped into the tank above the deepest point, which is also above the lower extremity of the conveyor. The active number of the conveyor is an endless belt driven so that the upper course of the belt moves upwardly to carry the ripe blueberries that have settled toward the bottom.
The movement of the upper course of the belt through the liquid tends to generate a circulation within the liquid that proceeds from the upper end of the conveyor back across the surface of the liquid, and downward toward the lower extremity of the conveyor. This primary circulation is often augmented by sprays of the liquid directed downward into the tank from above this position. Forcing the berries downward into the liquid has the desirable effect of increasing the velocity of the movement of the ripe berries downward to the point where they can be deposited on the upper course of the conveyor. The lighter weight green berries can then separate from the mass as the conveyor belt moves the accumulated collection on the inclined path upward toward the surface. Where the buoyancy differential is substantial, the green berries will begin to move toward the surface immediately without closely approaching the conveyor.
A problem with this type of device has been in the effect of the movement of the lower course of the conveyor at a downward incline through the liquid, tending to drag some of the liquid along with it to produce a counter-circulation to the primary circulation described above. This obviously tends to produce a random turbulence that interferes with the desired movement of the berries. Where the buoyancy differential is minimal, this turbulence can easily seriously impair the ability the machine to produce a sorting operation at a satisfactory rate of production. The present invention is directed to this problem.